Removable panels frequently are secured to a supporting structure by means of captive fasteners, typically including screws carried by the panel to engage nuts on the supporting structure. In the manufacture of some of these captive screws, the shank of the screw blank is extended through a collar, after which threads are rolled on the outer end of the shank, producing threads having a major diameter greater than the diameter of the opening in the collar. Therefore, the screw cannot escape from the collar. However, because of the diameter of the threads, the shank of the screw cannot be retracted into the collar when the fastener is in the separated condition. This means that the threaded screw shank end will project beyond the collar where it is subject to damage because it is not protected. For example, deck plates on ships periodically must be removed to provide access to the space below. These deck plates commonly are thrown down on the adjacent deck area upon their removal. Conventional protruding captive screws are unsuitable for such uses because they will become damaged and unusable when the plates are dropped and the impact is taken by the unprotected screw shank ends.
It is possible to increase the length of the collar and provide a counterbore of larger diameter to permit the threaded screw shank to retract fully into the counterbore area. This requires a significant increase in the length of the collar, however, which causes the collar to project a substantial distance beyond the surface of the panel to which it is secured. Again, taking the example of deck plates, this is unacceptable because the high projections of such elongated collars off the level of the deck.
Another problem with conventional fasteners arises from the fact that the nut is substantially fixed in position axially by an element of the assembly which retains it. This may be merely a flared end on the nut which forms a flange that engages the supporting structure to hold the nut in place against a force tending to push it away from the supporting structure. This is not a strong attachment, but that is not important if the fastener is used properly because significant push-out forces are not imposed on the nut. When the screw is tightened all the way into the nut, the head of the screw engages the seat on the collar, and the nut is drawn against the supporting structure rather than being pushed away from it.
However, if the screw is not fully tightened, the nut will hold the screw so that the screwhead is displaced from its seat. Therefore, if an axial load is imposed on the screw partially secured in this manner, such as by stepping or dropping something on it, the force will be transmitted directly to the nut. This may be much too great a load for the nut retention flange to absorb and, as a result, the nut will be popped loose from the supporting structure. This destroys the functionality of the fastener.